32.2: Ornithischians

Figure \(\PageIndex{1}\): Heterodontosaurus reconstruction with possible feather-like structures. (Drawing by FunkMonk via Wikimedia.)

The sister group to saurischia, the ornithischians are a diverse group of dinosaurs. Their hip, which is more flattened and pointed towards the tail, allows for a larger gastro-intestinal system which these animals used to process their almost exclusively plant-based diets. The first ornithischians were small bipedal runners, with Lesothosaurus claiming the title of oldest ornithischian and the Heterodontosaurs being the most famous members of this group. Like the name implies (hetero=different, dont=tooth, saurus=lizard), these animals had a unique trait that most reptiles do not, neither at that time nor even today: differentiated, specialized teeth. They had more rasping teeth toward the back of their mouths and more spiky teeth toward the front. They also had an extension of their jaw in front of the teeth. This would become a beak in its descendents, a feature that is common in many ornithischians.

Thyreophora

Stegosaurs

Figure \(\PageIndex{2}\): A, mounted skeleton in right lateral view and B, laid out with missing elements reconstructed before mounting. (Image by Susannah Maidment et al. & Natural History Museum, London)

There are two major subgroups within ornithischia: thyreophora and neornithischia. Thyreophora represent the group of armored dinosaurs, which had bony buildups protruding and covering the skin. The stegosaurs, one major branch of the thyreophora tree, were common in the Jurassic. Their bony plates on their back are still one of the biggest mysteries in paleontology. Various ideas, from thermoregulation, defense, and sexual display have been proposed, but none have been fully convincing. One thing to consider is related animals, like Kentrosaurus and Miragaia, had very different set ups. Any explanation of why Stegosaurus’ had its plates has to explain why others did not have them.

Figure \(\PageIndex{3}\): Cartoon and picture of a Stegosaurus/Allosaurus interaction. (Image care of Robert Bakker.)

Less confusing are the defensive tail spikes, which match holes in the bones of predatory dinosaurs at the time. Jokingly called a ‘thagomizer’ in a Far Side cartoon by Gary Larson in the 1980s, this name has now been somewhat accepted in the scientific community. One famous example of the thagomizer’s impact was first presented in 2014 by Robert Bakker and colleagues in which an Allosaurus had a hole in its groin that abscessed and probably killed the animal. The most likely cause of the hole is a thagomizer spike.

Ankylosaurs

Figure \(\PageIndex{4}\): Ankylosaurus reconstruction (by Emily Willoughby via Wikimedia.)

The other major group of thyreophorans are the ankylosaurs (Ankylosauria). These were heavily armored tank-like animals common in the Cretaceous. There are at least two major subdivisions in this category: the tail-clubbed Ankylosauridae and the more spiky Nodosauridae. Common depictions of Ankylosaurus, as in the Jurassic World films, show an unrealistic hybrid that had the shoulder spikes with the tail club, but in reality it was one or the other.

Early Ornithischians and Thyreophora: Did I Get It?

Exercise \(\PageIndex{1}\)

What feature do early Ornithischians like Heterodontosaurus have that they passed down to their descendants?

a. Clubs

b. Beak

c. Fangs

d. Spikes

Answer

b. Beak

Exercise \(\PageIndex{2}\)

Stegosaurus' tail spikes are called ________, named by cartoonist Gary Larsen.

a. sharpinator

b. stabifier

c. thagomizer

d. allofier

Answer

c. thagomizer

Exercise \(\PageIndex{3}\)

Which of the following would a nodosaur (and only a nodosaur) have?

a. Tail spikes

b. Tail club

c. Shoulder spikes

d. Tail without club or spikes

Answer

d. Tail without club or spikes

Ornithopods

Figure \(\PageIndex{5}\): Drawing of an Iguanodon skeleton by G. Lavette as it was found in 1882, nearly articulated.

The other side of the ornithischian tree (i.e., Cerapoda/Neornithischia) has three other major groups. The more complex, diverse, and prolific of the three are the ornithopods. Their name means ‘bird feet’ due to the three toes, but remember that the theropods (only distantly related) were the group that evolved into birds. Their success did not come from armor, speed, or size but something much more mundane: chewing. Their tooth battery would remind one of a modern horse or cow; highly complex (for the time) and effective at breaking down tough plants. Like other groups, they started small and bipedal (like the hypsilophodonts), but eventually grew larger. The large ornithopods are speculated to be somewhere between a biped and a quadruped like a chimpanzee or gorilla. A famous member of this group that is transitional between the early and later ornithopods is Iguanodon. Though quite large, its thumb spike and prehensile pinky must have had some purpose, suggesting it had to spend some time on two legs.

Figure \(\PageIndex{6}\): Parasaurolophus skull with a view into nasal cavity of the cranial crest. It is hypothesized that the Parasaurolophus pushed air through its long cranial crest to make low register sounds that could be heard for miles. 1. Nostril 2. Sinus cavity 3. Skin 4. Intra Cranial bone 5. Skull 6. Cranial crest

When the average person thinks of the animals in this category, typically the word they would use to describe them would be ‘hadrosaur’ or ‘duck-billed dinosaur.’ These terms are appropriate, but only for the more derived members of the group. So while Iguanodon is not a hadrosaur, other famous ornithopods like Parasaurolophus and Edmontosaurus are hadrosaurs. The hadrosaurs break into two major groups: the saurolophines (formerly known as hadrosaurine) which had no crest or solid crests and the lambeosaurines which had hollow crests. One famous member of saurolophines is Maiasaura, the first dinosaur found with definitive evidence of parenting, hence the name, ‘good mother lizard.’ The young of Maiasaura were found to have worn teeth before they had the ability to walk, suggesting the mother had to have brought food to the brood. Parasaurolophus is a famous member of the lambeosaurines. Their hollow crests have been speculated to allow them to make sounds, presumably to communicate with members of their own species.

Ceratopsians

Figure \(\PageIndex{7}\): Drawing of Protoceratops (by AntoninJury via Wikimedia.)

The sister group to the ornithopods is marginocephalia. These animals also had tooth batteries, but also had head ornamentation. Marginocephalia has two major subdivisions. The first is the ceratopsians, or horn-faced dinosaurs. It should be noted that despite the name, not every member of this group had a horn on their face. What was much more universal was a frill, a bony projection that extended from the skull back towards the neck. Early members of the group were from Asia and were small with tiny frills. A famous early example is Protoceratops from Mongolia. The name, which means ‘before horned face’, shows it had the neck frill and just the smallest bumps where horns evolved in others.

Figure \(\PageIndex{8}\): Triceratops skeleton from the Los Angeles Museum of Natural History. (Photo by Allie_Caulfield edited by MathKnight via Wikimedia.)

Within the ceratopsian family, there are two subfamilies. The chasmosaurs typically had more trapezoidal frills with larger brow horns. The most famous member of this group is Triceratops. The centrosaurs typically had larger nose buildups (horns and sometimes bulbous bony growths) and spikes on the frill. Styracosaurus is a famous example of a centrosaur. There is still much debate as to the purpose of the horns, spikes, and frills. Popular convention interprets these items as defenses from predators. While there is some evidence for this, not all scientists agree that they were defensive, especially the frills. Most ceratopsians’ frills were weak and thin, especially in the centers of each half, making them useless against direct combat. Other ideas for their growth include sexual display and species recognition. This goes along with many fossil deposits of ceratopsians that show many individuals in the same place of different ages, which suggests herding or other social behavior.

Pachycephalosaurs

Figure \(\PageIndex{9}\): Sketch of a Pachycepholosaur from the Hell Creek Formation of South Dakota (by Jordan Mallon via Wikimedia.)

The other branch of marginocephalia is the pachycephalosaurs, a name that means ‘thick headed lizards.’ They were bipedal and smaller than many other dinosaur types, with the largest animals just reaching the height of a grown human. Their head buildups did not result in frills or true horns, but a dome head surrounded by small spikes. This dome is often suggested to be for intra-species combat over mates or territory, similar to what rams or deer do today. The evidence of this, like so many other things in the fossil record, is circumstantial and incomplete. Their domes could have been used for similar things as their ceratopsian cousins: mating and recognition. Another complication scientists have between the pachycephalosaurs is naming and differentiating them. It has been proposed that many individual genera may be invalid, with these variations proposed to instead reflect differences in age or gender. Stigymoloch and Dracorex, for example, have been proposed as the subadult and juvenile versions of Pachycephalosaurus, respectively.

Cerapoda: Did I Get It?

Exercise \(\PageIndex{1}\)

Hadrosaurs are split based on their ______.

a. feet

b. teeth

c. crests

d. tails

Answer

c. crests

Exercise \(\PageIndex{2}\)

Where did ceratopsians start?

a. Asia

b. Eastern US

c. Africa

d. Western US

Answer

a. Asia

Exercise \(\PageIndex{3}\)

Many pachycephalosaurs may actually be what instead of separate species?

a. Lack of complete fossils

b. Different positions in the pack

c. Evolutionary changes over time

d. Different ages

Answer

d. Different ages